The manufacture of precision gears for drive trains, e.g., helicopter rotor transmissions, involves multiple highly-controlled fabrication steps which necessitate the use of highly-sophisticated manufacturing equipment, e.g., cutting apparatus, carburizing vessels, quenching equipment, etc., and highly-skilled operators to perform each fabrication step. As such, precision gears are amongst the most complex and costly articles of manufacture to fabricate. The elimination or simplification of a single process step, or a process improvement which eliminates or reduces the number of rejected or scrapped workpieces, can produce significant fiscal benefits.
FIGS. 1a-1f pictorially illustrate various stages of fabricating a precision gear utilizing conventional manufacturing techniques. For simplicity, a small segment of the precision gear is shown, i.e., a segment corresponding to two gear teeth, but it should be understood that the entire precision gear is identically-formed. FIG. 1a depicts a steel gear blank or forging 102 having a thin layer of copper plate 104 deposited thereon. In a prior step, the steel forging 102 has undergone a conventional copper electro-plating process wherein the copper plate 104 has been deposited to a minimum thickness of about 0.0008 inches (0.0020 cm). As will be appreciated in the subsequent discussion and views, the copper plate 104 serves to mask predefined areas of the precision gear 100 (FIG. 1f) from exposure to one or more subsequent carburization cycles.
In FIG. 1b, the gear teeth 106 are rough-machined utilizing a standard reciprocating shaper-cutter 108 which mills the profile of the gear teeth 106, e.g., the drive and coast flank involutes and the fillet radius between each gear tooth 106. Such rough machining operation mills the gear tooth profile to within about 0.010 inches (0.0254 cm) of its final dimensions.
In FIG. 1c, an abrasive wheel cutter 109 is employed to chamfer and deburr the edges 110 of the gear tooth profile. Such chamfering operation serves to minimize stress concentrations in the completed precision gear 100.
As a result of the prior machining operations, the copper plate 104 remains in areas corresponding to the top land 112 and end faces 114 of each gear tooth 106. Yet another consequence of the machining operations, is the inadvertent removal of copper plate, shown as void areas 116 in FIGS. 1c and 1d, due to handling prior to and during such machining operations. In FIG. 1d, a delicate operation is performed to "touch-up" these unplated areas 116 with a carbon stop-off paint such as produced by Park Chemical Company under the tradename "NO-CARB". Such carbon stop-off paint is functionally equivalent to the copper plate 104 inasmuch as it serves to mask these unplated areas 116 from exposure during at least one subsequent carburization cycle.
In FIG. 1e, the machined/masked workpiece 118 has undergone a conventional carburization cycle wherein atomic carbon diffuses into the exposed surfaces of the gear teeth 106, e.g., the flanks 120, fillets 122, and chamfered edges 110 thereof. More specifically, the workpiece 118 is heated to an elevated temperature (i.e., about 1650-1800 degrees F, 899-982 degrees C) and placed in an atmosphere rich in carbon monoxide or hydrocarbon gases for a period of about 4 hours. During this process, the exposed surfaces 120, 122, 110 of the gear teeth 106 absorb atomic carbon to a depth of about 0.030 inches (0.076 cm) to about 0.060 inches (0.152 cm) while the copper plate 104 inhibits the absorption of carbon into the top lands 112 and end faces 114 of the precision gear 100. As such, the carburized areas, following a subsequent hardening step, provide a hard, wear-resistant surface while the uncarburized areas ensure that the core of the gear remains comparably soft to improve the toughness and durability of the precision gear 100.
In FIG. 1f, the precision gear 100 is shown in its finished form after having undergone several operations including tempering, copper stripping, heat treat/quenching, and/or final machining. The tempering operation involves heating the workpiece to an elevated temperature of about 1100 degrees F (593 degrees C) for a period of about 2 hours. Such tempering operation, which is performed following the carburization cycle and/or hardening operation, relieves residual stresses which develop as a result of the preceding operations. The copper stripping operation includes the step of chemically stripping the copper plate from the top lands 112 and end faces 114 of the workpiece in a cyanide bath. This operation may be viewed as an antithetical operation to the copper electro-plating process insofar as the polarity of the precision gear is reversed, i.e., is the anode in the electric circuit, to remove the copper plate. The heat treat/quenching operation includes the steps of elevating the temperature of the in-process workpiece to about 1650-1800 degrees F and rapidly quenching the heated workpiece in a cool oil. Such heat treat/quenching transforms the steel microstructure from austenite to martinsite. Insofar as the prior carburizing cycle locally increases the carbon content along the surfaces of the flanks 120 and fillets 122 of the gear teeth 106, the heat treat/quenching operation produces an extremely hard, wear resistant shell or "case" and a comparably ductile interior core. This combination improves the fatigue properties of the precision gear 100. The final operation involves machining the workpiece to its final dimensions. This step is generally performed utilizing a Cubic Boron Nitride (CBN) cutter having a shape corresponding the tooth space profile, i.e., the profile defined by and between two adjacent teeth 106.
The prior art manufacturing method presents certain fiscal and structural disadvantages. Firstly, the touch-up operation, shown in FIG. 1d, is a corrective step rather than a value-added step. That is, the touch-up operation corrects for the adverse consequences of prior machining/handling operations, and, accordingly, increases cost without adding benefit.
Secondly, the touch-up operation is painstakingly laborious and requires the skills of an artisan to ensure that all unplated areas have been addressed and/or that the carbon stop-off paint has not inadvertently spilled or run-off on surfaces to be carburized. Should the operator inadvertently overlook an unplated area 116, for example, along a top land 112 of a gear tooth, a local, high concentration of carbon will be diffused into the top land 112 during the carburization cycle. As such, the tip of the gear tooth becomes highly brittle following the heat treat/quenching operation and the hardened tip may result in "tooth capping" or "case-core separation". In yet another example, should the operator inadvertently spill the carbon stop-off paint on the flank 120 of a gear tooth, a local "soft-spot" will develop along the surface. As such, the gear tooth may spaul in this area when in operation. In either event, the precision gear 100 may fail prematurely, or, depending upon the severity of the defect, may require rework or be scrapped.
Finally, the chamfered edges 110 produced by the deburring/chamfering operation, shown in FIG. 1c, can also be a source of tooth capping insofar as a high carbon content can develop in the comers 110.sub.C of the chamfered edges 110. While the deburring/chamfering operation has the adverse affect of removing copper plate from these areas, it is desirable to perform such operation prior to carburization and/or heat treat/quenching when the precision gear is relatively malleable and easily machined. While hardening of the chamfered edges 110 could be avoided with the use of a carbon stop-off paint, such operation is typically deemed impractical based on the laborious nature of the touch-up operation. Furthermore, such operation produces an unacceptably high risk of error based on the probability that inadvertent spillage onto surfaces to be carburized is more likely to occur.
Accordingly, there is a constant search in the art for manufacturing tools and processes which eliminate or simplify fabrication steps, diminish the potential for fabrication errors, and improve the structural properties of a precision gear.